The majority of fuel today is derived from crude oil. Crude oil is in limited supply, and fuel derived from crude oil tends to include nitrogen-containing compounds and sulfur-containing compounds, which are believed to cause environmental problems such as acid rain.
Natural gas is abundant and may be converted into hydrocarbon fuels, lubricating oils, chemicals, and chemical feedstocks. One method for producing such products from natural gas involves converting the natural gas into synthesis gas (“syngas”) which is a mixture primarily of hydrogen and carbon monoxide. In the Fischer-Tropsch process, the syngas produced from a natural gas source is converted into a product stream that includes a broad spectrum of products, including gases, such as, propane and butane; a liquid condensate which may be processed into transportation fuels; and wax which may be converted into base oils as well as lower boiling products, such as, diesel. The conversion of the wax and condensate usually involves passing the feed downwardly along with a co-current hydrogen enriched gas stream through a catalyst bed contained in one or more hydroprocessing reactors (i.e., a downflow reactor). The liquid hydrocarbon feed “trickles” down through the catalyst beds in the hydroprocessing reactor and exits the reactor bottom after the desired upgrading is achieved.
The Fischer-Tropsch feed stream as recovered from the Fischer-Tropsch reactor may contain filterable particulate contaminants, such as, for example, catalyst fines and rust and scale derived from the equipment. In addition, in some instances, un-filterable aluminum-containing contaminants have been found in the feed stream which cannot be removed using conventional particulate recovery methods. These un-filterable aluminum contaminants will coalesce into particulates under the conditions prevailing in the hydroprocessing reactor and can cause serious operating difficulties in a fixed-bed, trickle-flow hydroprocessing reactor. The most frequent difficulty is pressure drop build-up and eventual plugging of the flow-paths through the catalyst beds as the catalyst pellets filter out the feed particulates. Such build-up can cause significant economic loss in lost production and replacement catalyst costs. These non-filterable aluminum-containing contaminants usually will concentrate in the heavier wax fraction of the Fischer-Tropsch product stream. U.S. Pat. No. 6,359,018 describes an upgrading process in which the Fischer-Tropsch feed stream passes in up-flow mode through the hydroprocessing reactor and is then filtered to remove the particulates.
There are two types of up-flow operation which may be used in carrying out the present invention, fixed bed and ebullating bed operation. When a fixed bed reactor is operated in up-flow mode, there is little or no expansion of the catalyst bed during operation. It should be understood that since the reactor walls are rigid, the expansion of the catalyst bed will take place only along the vertical axis of the bed. Thus, when referring to bed expansion in this disclosure, the increase in height of the bed or depth of the bed in the reactor is an appropriate measure of bed expansion and is directly related to volume. An ebullating bed also employs the upward flow of feedstock, however, an ebullating bed differs from an up-flow fixed bed in that the upward flow in the ebullating bed is sufficient to suspend the catalyst and create random movement of the catalyst particles. During operation the volume of an ebullating bed will expand, usually by at least 20 percent, as compared to the volume of catalyst in the reactor when there is no flow of hydrogen and feedstock through the bed.
Up-flow fixed bed operation and ebullating bed operation differ from fluidized bed operation which is not used in the carrying out the present invention. In fluidized bed operation finely divided solid catalyst particles are lifted and agitated by a rising stream of process gas. In a fluidized bed the catalyst particles are suspended or entrained in the rising gas stream. A fluidized bed is sometimes referred to as a boiling bed due to its appearance to a boiling liquid. Bed expansion in a fluidized bed is considerably greater than observed in an ebullating bed.
It would be advantageous to provide an efficient process for removing both the filterable and un-filterable contaminants from the Fischer-Tropsch feed stream prior to the downstream hydroprocessing operations. The present invention provides such a process.
As used in this disclosure the word “comprises” or “comprising” is intended as an open-ended transition meaning the inclusion of the named elements, but not necessarily excluding other unnamed elements. The phrase “consists essentially of” or “consisting essentially of” is intended to mean the exclusion of other elements of any essential significance to the composition. The phrase “consisting of” or “consists of” is intended as a transition meaning the exclusion of all but the recited elements with the exception of only minor traces of impurities.